Water heater tanks are normally formed of steel and to prevent corrosion of the steel tank, corrosion resistant coatings, such as glass or porcelain enamel are applied to the inner surface of the tank. However, it has been found that intensified corrosion can occur in areas of the steel tank exposed through defects in the glass coating, or in inadequately coated portions of the steel tank. Further, steel fittings or connectors that may be exposed to the water of the tank are also subject to corrosion. To prevent this corrosion, sacrificial anodes formed of a metal electro-positive to steel, such as aluminum, magnesium, zinc, or alloys thereof, are normally mounted within the tank. The anode is electrically connected to the steel tank and a galvanic circuit is established which will preferentially corrode the anode.
The service life of the anode is dependent on the magnitude of the galvanic current generated in the cathodic protection system. In certain water supplies having a high mineral content, the current flow will be relatively high, resulting in a corresponding increase in the consumption of the anode and a corresponding decrease in the service life of the anode.
In order to reduce the consumption of the anode in a water heater tank, it has been proposed to connect a resistor in the galvanic circuit to limit the amount of current flow and, therefore, extend the service life of the anode. For example U.S. Pat. No. 4,093,529 shows a disc-like resistor, while other anode assemblies have included resistors in the form of wires or coils, as disclosed in U.S. Pat. Nos. 2,656,314, 2,568,594, 2,616,844, and 2,486,871. In addition, it has been proposed to employ a metal silicon oxide semiconductor in the galvanic circuit to act as a current control element, as disclosed in U.S. Pat. No. 4,381,981.